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Chapter 8 Lecture
Astronomy: A Beginner’s
Guide to the Universe
Seventh Edition
Moons, Rings,
and Plutoids
© 2013 Pearson Education, Inc.
Chapter 8 Moons, Rings, and Plutoids
© 2013 Pearson Education, Inc.
8.1 The Galilean Moons of Jupiter
All four Jovian planets have extensive moon
systems, and more are continually being
discovered.
The Galilean moons of Jupiter are those observed
by Galileo: Io, Europa, Ganymede, and Callisto.
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8.1 The Galilean Moons of Jupiter
This image shows Jupiter with two of its Galilean
moons.
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8.1 The Galilean Moons of Jupiter
The Galilean
moons and their
orbits
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8.1 The Galilean Moons of Jupiter
Their interiors
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8.1 The Galilean Moons of Jupiter
Io is the densest of Jupiter’s moons, and the
most geologically active object in the solar
system.
• It has many active volcanoes, some quite
large.
• Io can change surface features in a few
weeks.
• Io has no craters; they fill in too fast – Io has
the youngest surface of any solar system
object.
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8.1 The Galilean Moons of Jupiter
Io is very close to Jupiter, and also
experiences gravitational forces from
Europa. The tidal forces are huge, and
provide the energy for the volcanoes.
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8.1 The Galilean Moons of Jupiter
Europa has no craters;
surface is water ice, possibly
with liquid water below.
Tidal forces stress and crack
ice; water flows, keeping
surface relatively flat.
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8.1 The Galilean Moons of Jupiter
Ganymede is the largest
moon in the solar system –
larger than Pluto and
Mercury.
It has a history similar to
Earth’s Moon, but with water
ice instead of lunar rock.
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8.1 The Galilean Moons of Jupiter
Callisto is similar to Ganymede; no evidence of
plate activity.
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8.2 The Large Moons of Saturn and Neptune
Titan has been known for many years to have an
atmosphere thicker and denser than Earth’s;
mostly nitrogen and argon.
Makes surface
impossible to see; the
picture at right was
taken from only 4000 km
away.
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8.2 The Large Moons of Saturn and Neptune
Infrared image of Titan,
showing detail, and
possible icy volcano
Few craters, consistent
with active surface
Complex
chemical
interactions in
atmosphere
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8.2 The Large Moons of Saturn and Neptune
The Huygens lander took these
images of the surface of Titan.
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8.2 The Large Moons of Saturn and Neptune
Trace chemicals in
Titan’s atmosphere
make it chemically
complex.
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8.2 The Large Moons of Saturn and Neptune
Triton is in a retrograde orbit;
its surface has few craters,
indicating an active surface.
Nitrogen geysers
have been observed
on Triton,
contributing to the
surface features.
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8.3 The Medium-Sized Jovian Moons
Densities of these moons suggest that they are
rock and water ice.
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8.3 The Medium-Sized Jovian Moons
Moons of Saturn, in natural color
Note the
similarities,
as well as
the large
crater on
Mimas.
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8.3 The Medium-Sized Jovian Moons
Moons of Uranus and Neptune
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8.3 The Medium-Sized Jovian Moons
Miranda shows evidence of a violent past,
although the origin of the surface features
is unknown.
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8.4 Planetary Rings
The ring system
of Saturn is
large and
complex, and
easily seen from
Earth. The other
Jovian planets
have ring
systems as well.
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8.4 Planetary Rings
The rings are not solid; they are composed of
small rocky and icy particles.
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8.4 Planetary Rings
Our view of Saturn’s
rings changes as the
planets move in their
orbits.
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8.4 Planetary Rings
The Roche limit
is where the
tidal forces of
the planet are
too strong for a
moon to
survive; this is
where rings are
formed.
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8.4 Planetary Rings
All observed ring systems are within this limit.
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8.4 Planetary Rings
Voyager probes showed Saturn’s rings to be
much more complex than originally thought.
Earth is shown
on the same
scale as the
rings.
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8.4 Planetary Rings
“Shepherd” moons
define the edges of
some of the rings.
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8.4 Planetary Rings
Jupiter has been found to have a small, thin ring.
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8.4 Planetary Rings
The rings of
Uranus are more
complex.
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8.4 Planetary Rings
Two shepherd
moons keep
the epsilon ring
of Uranus from
diffusing.
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8.4 Planetary Rings
Neptune has five rings, three narrow and two
wide.
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8.5 Beyond Neptune
Pluto was discovered
in 1930. It was
thought to be needed
to explain
irregularities in the
orbits of Uranus and
Neptune, but it turned
out that there were no
such irregularities.
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8.5 Beyond Neptune
Pluto’s moon, Charon, was discovered in 1978.
It is orbitally locked to Pluto, and about a sixth
as large.
Pluto also has two smaller
moons, Nix and Hydra,
discovered in 2005.
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8.5 Beyond Neptune
Charon’s orbit is at a large angle to the plane of
Pluto’s orbit.
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8.6 Plutoids and the Kuiper Belt
The first Kuiper belt
objects were observed
in the 1990s, and more
than 1200 are now
known. Some of them
are comparable in size
to Pluto.
These images show
Eris and its moon
Dysnomia.
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8.6 Plutoids and the Kuiper Belt
This figure shows several of the largest known
trans-Neptunian objects, now collectively called
plutoids.
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Summary of Chapter 8
• Outer solar system has 6 large moons, 12
medium ones, and many smaller ones.
• Titan has a thick atmosphere and may have
flowing rivers of methane.
• Triton has a fractured surface and a retrograde
orbit.
• Medium-sized moons of Saturn and Uranus are
mostly rock and water ice.
• Saturn’s rings are complex, and some are
defined by shepherd moons.
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Summary of Chapter 8, cont.
• The Roche limit is the closest a moon can
survive near a planet; inside this limit rings
form instead.
• Jupiter, Uranus, and Neptune all have faint
ring systems.
• Pluto has three moons, Charon, Nix, and
Hydra.
• Dwarf planets beyond Neptune (including
Pluto) are now known as plutoids.
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